Nonmonotonic Ionic Transport and Its Correlation with Structural and Dynamic Heterogeneities in NaTFSI and...

Sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) salt-based water-in-salt electrolytes (WiSEs) hold significant advantages over their Li ion-based counterparts. Experimentally, the ionic conductivities of the aqueous solutions of these salts show nonmonotonic behavior with the salt concentration. Here, we aim to explain the possible origin of the nonmonotonic behavior of the observed ionic conductivities of both the electrolytes with the aid of atomistic molecular dynamics simulations. Our in-depth analysis reveals that the observed maximum in the conductivities, at 6 m concentration for NaTFSI and 5 m for NaFSI-based electrolytes, could be due to the presence of a more dynamic solvation environment of the ions, where frequent changes in the number of ions around them are observed. Furthermore, by analyzing the average sizes of the Na⁺ ion solvation shells, comprising water and anions, and the mean diffusion lengths of the Na⁺ ions traveled through water and anionic regions, we find that the Na⁺ ions adapt a mixed (vehicular as well as structural exchange) transport mechanism only at lower concentrations of NaFSI salt because of their dominant interaction with water molecules, but they follow a structural exchange mechanism of transport at higher concentrations due to their strong interaction with the FSI^- anions. On the contrary, for NaTFSI salt, water molecules are found to promote mixed transport mechanism for all concentrations because of the strong Na⁺-water interaction. However, when Na⁺ ions encounter TFSI^- anions, their movement can proceed through a structural exchange mechanism. The study also reveals the presence of enhanced dynamic heterogeneity in both electrolytes in their highly concentrated regions.